Stereotactic Body Radiation Therapy for Prostate Cancer After Radical Prostatectomy: First Toxicity Results of a Prospective Single-Arm Phase 2 Trial (SCIMITAR)

Committee/Faculty

Presenting Author(s)

TM

Ting Martin Ma, n/a

MD, PhD
UCLA
Los Angeles, California, United States

Disclosure: I do not have any relevant financial / non-financial relationships with any proprietary interests.

Co-Author(s)

LB

Leslie Ballas

MD
USC
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
AS

Ankush Sachdeva

BS
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
NC

Natalie Chong

BS
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
TY

Tiffany Yang

n/a
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
VB

Vincent Basehart

BS
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
RR

Robert Reiter

MD
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
CS

Christopher Saigal

MD
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
SY

Stephanie Yoon

MD
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
YG

Yu Gao

PhD
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
MS

Michael Steinberg

MD
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
MC

Minsong Cao

PhD
UCLA
Los Angeles, California, United States

Disclosure: Disclosure information not submitted.
Amar U. Kishan, MD

MD
UCLA
Los Angeles, California, United States

Disclosure: Varian Medical Systems (Individual(s) Involved: Self): Consultant/Advisory Board, Speaker/Honoraria; ViewRay (Individual(s) Involved: Self): Consultant/Advisory Board, Ownership Interest, Research Grant, Speaker/Honoraria

Purpose: To report initial toxicity results from a prospective phase 2 trial of stereotactic body radiotherapy (SBRT) to the prostate bed ± nodes after radical prostatectomy (RP) (NCT03541850).

Methodology: 100 patients with clinically-localized, node-negative prostate cancer after RP with either adverse pathologic features (positive margin, pT3-4, pathologic Gleason score 8-10 or tertiary Gleason grade 5), or ≥2 consecutive rising prostate-specific antigen (PSA) with PSA>0.03 ng/mL, or a Decipher score>0.45 were enrolled. Prostate bed SBRT (median 32Gy/5 fractions) was delivered with either CT guidance (CTgRT) or MRI guidance (MRgRT). Simultaneous integrated boosts (SIBs) to gross disease and/or elective nodal radiation were optional per physician discretion, as was a 6-month course of androgen deprivation therapy (ADT). Acute (within 90 days of SBRT) and late (90-180 days of SBRT) genitourinary (GU) and gastrointestinal (GI) toxicity were evaluated based on the Common Terminology Criteria for Adverse Events (CTACE version 4.03). Patient-reported toxicity outcomes (PRO) were based on Expanded Prostate Cancer Index-26 (EPIC-26) instrument and International Prostate Symptom Scores (IPSS).

Results: Median follow-up was 18 months. Sixty-nine patients received CTgRT and 31 received MRgRT. Median interval from RP to SBRT was 22.8 months. Median dose to the prostate bed was 32Gy (IQR 30-34Gy) in 5 fractions. 27%, 27%, and 41% of patients received SIBs to gross disease, elective nodal radiotherapy and ADT, respectively. Toxicity data in Table 1. Acute and late Grade 2 GU toxicity was 9% and 9%, respectively. There was one (1%) grade 3 GU toxicity (hematuria) within the first six months. Acute grade 2 GI toxicity was 5%, with 1 case (1%) of grade 3 toxicity (rectal fistula). There was one case (1%) each of grade 2 and grade 3 late GI toxicity. No patient treated with MRgRT experienced any grade 3 GU/grade≥2 GI toxicity. Compared to CTgRT, MRgRT had a 30.5% (95%CI 11.6-49.5%) reduction in any grade acute GI toxicity (p=0.0056 by chi-square) and a 32% (95%CI 12.9-51.1%) reduction in any grade cumulative GI toxicity up to 6 months (p=0.0021). The low incidence of acute and late GU and GI toxicities were confirmed by PRO data.

Table 1. Toxicity Results
CTgRT (n=69) MRgRT (n=31)
Acute Late Acute Late
% % % %
Highest GU toxicity grade
0 44.9 46.4 51.6 58.0
1 45.0 43.5 38.7 32.3
2 8.7 8.7 9.7 9.7
3 1.4 1.4 0 0
Highest GI toxicity grade
0 27.5 62.3 58.1 71.0
1 63.9 36.3 41.9 29.0
2 7.2 0 0 0
3 1.4 1.4 0 0

Conclusions: SBRT to the prostate bed after RP was feasible with favorable toxicity profile, both in clinician- and PRO measurements. MRgRT may be associated with reduced GI toxicity. Primary endpoint analysis planned once the 100th patient has 2-year PRO data available.

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